fix password resetting

src/Jackett/Controllers/ServerConfigurationController.cs

@@ -47,7 +47,7 @@ namespace Jackett.Controllers.V20
         {
             var oldPassword = serverConfig.AdminPassword;
             if (string.IsNullOrEmpty(password))
-                password = string.Empty;
+                password = null;
 
             if (oldPassword != password)
             {

src/Jackett/Services/SecuityService.cs

@@ -24,6 +24,8 @@ namespace Jackett.Services
 
         public string HashPassword(string input)
         {
+            if (input == null)
+                return null;
             // Append key as salt
             input += _serverConfig.APIKey;